1. Field of the Invention
The invention relates to a method for calibrating sensors, in particular turbidity sensors in domestic appliances, and a relevant domestic appliance for carrying out the method.
2. Description of Related Art
In domestic appliances, e.g. dishwashers or machine machines, turbidity sensors are used to determine the degree of contamination of the cleaning fluid, e.g. washing solution or washing liquid. Values of the degree of contamination determined by the turbidity sensor are used for further control of the cleaning program of the domestic appliance. In a dishwasher the cleaning program consists, for example, of the partial program steps “pre-wash”, “clean”, “intermediate rinse”, “clear rinse” and “dry”. Frequently a plurality of intermediate rinsing steps are carried out within the partial program step “intermediate rinse”. By using the values of the degree of contamination determined by the turbidity sensor, the dishwasher controller can discontinue the execution of further intermediate rinsing steps when the degree of contamination falls below a certain value. Thus, a considerable saving of water and energy can be achieved with the same cleaning results. In addition, if the degree of contamination is low during the “pre-wash”, the washing liquid from the “pre-wash” can be used for the “clean” partial program step.
The turbidity is generally measured by passing light through the cleaning liquid. However, other physical measurement methods, e.g. using sound, are also feasible. When using the physical principle of passing light through the cleaning liquid, where particles pressed as a suspension in the cleaning liquid retain a part of the light, a transmitting and receiving device for the light is required. The transmitting device, for example, comprises a lamp or a light-emitting diode and the receiving device, for example, comprises a phototransistor. However, the transmitting and receiving devices are subjected to changes from usage and ageing. In addition, in some cases considerable deposits can occur on the optical devices. Temporary impurities on the transmitting and receiving devices can lead to appreciable errors in the measurements. In the course of time, this results in successively increasing errors in the measurements of the turbidity of the cleaning liquid. This gives rise to errors in the control of the domestic appliance.
Known from EP 0 862 892 B1 is a domestic appliance with a measuring device for determining the degree of contamination of a cleaning liquid. In order to prevent incorrect measurements, an adjusting measurement is carried in a cleaning program in a preceding cleaning program in which the measuring device is used to determine the degree of contamination of the cleaning liquid, this preferably being carried out in a program part with uncontaminated washing liquid, e.g. clear rinse. The measured value for the adjustment of the measuring device in the following cleaning program can be stored in a non-volatile memory. A disadvantage here is that if little intermediate rinsing is carried out or this is faded out, the rinsing solution can contain appreciable impurities during the clear rinsing so that the measurement results can be falsified. Furthermore, only one adjusting measurement is made so that in the event of randomly occurring severe contamination, e.g. caused by localized deposits on the transmitting device, measured values for the adjustment of the measuring device with considerable errors are the consequence.
A method for adjusting a turbidity sensor is known from DE 101 11 006 A1. Several calibration value measurements are made at different times within a wash program and stored in a first memory table, calibration value measurements being made in several wash programs. From these calibration value measurements, the calibration measured value having the lowest degree of contamination is determined by selection for each wash program and is written in a second memory table. The average is calculated from the stored selected calibration measured values of the second memory table and this forms the reference value for the measurement using the turbidity sensor.
A disadvantage is that only a relatively small number of calibration measurements forms the basis for determining the reference value which is merely the average of a plurality of individual measurements within a wash program. Consequently, sources of error which occur in several wash programs or only within an entire wash program, e.g. contamination on the optics of the transmitting device, cannot be identified. As a result of determining the reference value by merely averaging from all the calibration measured values for each wash program, these calibration measured values frequently loaded with considerable errors are disadvantageously included in the averaging. For example, if contamination occurs in the three preceding wash programs and this contamination is eliminated again in a following wash program, the measurement is nevertheless made using the reference value from the average of the frequently defective individual measurements, whereby this error is propagated until all the calibration measurements forming the basis for the reference value are not longer affected by errors caused by temporary impurities.